The present invention relates to an optical medium recording apparatus and an optical medium recording method, and more particularly to an apparatus and a method in which an information signal is recorded on an optical medium using a mark length recording method.
An optical disk apparatus, in which an optical disk is used as an optical medium because of its large storage capacity and medium interchangeability, is used in a filing system for storing image data or as an external storage device of a computer capable of code data recording/reproducing, The optical disk is being used in rapidly increasing number of different fields nowadays as a medium of supplying software or as a back-up device because it is mass-reproducible from a master disk, such as a compact disk.
Variations of such an optical disk used in recording/reproducing in an optical disk apparatus area: a read-only medium, a typical example of which is a CD-ROM; a write-once medium capable of recording only once and used mainly in a filing apparatus handling image information; and a rewritable medium capable of handling coded information and of recording and erasing information as many times as desired.
As an example of the rewritable medium above, a 5-inch magneto-optic disk, whose medium format is standardized by ISO, an international standardizing organization, is already available as a commercial product. After the 5-inch medium format was standardized, the 3.5-inch medium format was standardized by ISO, and several types of 3.5-inch optical disks are already available as a product.
In order for the 5-inch and 3.5-inch optical disks to become widely used in the future, it is essential that such disks have an even greater performance ability and larger storage capacity, as well being less expensive. In this respect, a recording method and a recording apparatus, capable of recording desired information accurately utilizing a high-density recording method, is necessary.
FIG. 1 illustrates the configuration of an example of an optical head in the conventional optical disk apparatus. In an optical head 11 of FIG. 1, a light beam emitted from a semiconductor laser 12 provided as a light source is turned into parallel light by a collimating lens 13, allowed to pass through a first beam splitter 14, reflected by a reflecting mirror 15, and caused to be incident on an objective lens 16.
The objective lens 16 converges the incident light beam and forms, by focusing the beam, a light spot on the medium surface of an optical disk 17 that rotates on a spindle, which is a rotating mechanism. The objective lens 16 itself is positionally controlled by means of a signal obtained by allowing an output control signal from an amplifier 22 described later to undergo signal processing, so that a minute spot narrowed to a diffraction limit is impinged accurately on a specified track while keeping track of radial runout and axial deflection of the magneto-optic disk 17. The optical head 11 is moved in a radial direction of the magneto-optic disk 17 by a moving mechanism 10.
When recording or erasing information on a medium, only the part thereof where a laser spot is impinged undergoes temperature rise beyond the Curie temperature, due to the irradiating laser power being magnified; and an external magnetic field applied by a bias coil 18 to the magneto-optic disk 17 causes the medium to be magnetized in the same direction as the externally applied magnetic field.
When reproducing information, signals on the recording medium are detected by utilizing a reflection of light from the magneto-optic disk 17. Light reflected by the magneto-optic disk 17 and converged by the objective lens 16 is allowed to travel on the same optical path that it travels on when it is incident on the disk, is then reflected by the first beam splitter 14 that the light passed through on its way to the disk, and caused to digress from the incident light path. The reflected laser light separated off by the first beam splitter 14 is split into a transmitted light and a reflected light by a second beam splitter 19. The reflected light is applied to a photodetector 21 and the amplifier 22 so as to generate an error signal for focus control and track control needed for the objective lens 16 to maintain accurate tracking.
The transmitted light from the second beam splitter 19 is split by a polarizing beam splitter 24 after passing through a wavelength plate 23, the split light beams being caused to be incident on photodetectors 27, 28 via converging lenses 25, 26, respectively. The output electric signals from the photodetectors 27, 28 are fed to the inputs of a differential amplifier 29 and an additional amplifier 30, respectively. The differential amplifier 29 generates a magneto-optic signal (MO signal); and the additional amplifier 30 generates an ID signal. The ID signal is a signal recorded as heights and depressions on a track, and includes a track number and a sector number. This ID signal is detected as a change in light amount accompanying diffraction of light by the heights and depressions on the recording medium. The magneto-optic signal is detected as a change in light polarization which depends upon the direction of magnetization.
FIG. 2 illustrates the construction of the magneto-optic disk medium. The magneto-optic disk 17 is constructed such that a recording layer 32 is formed on a substrate 31, and pre-grooves 33, used as grooves for guiding the light Spot so that it hits a specified track, are formed on the entire radial extent at a track pitch of 1.6 .mu.m. The depth of these pre-grooves 33 is preset to be about 1/4 of a wavelength .lambda. of a light beam so that the sensitivity of a track error signal is at a maximum.
Land portions 34 are formed between the adjacent pre-grooves 33. A minute spot obtained by converging the beam by means of the objective lens is impinged on this land portion 34. Pits or marks for the ID signal containing the track number and sector number are built in the form indicated by a numeral 35 in FIG. 2, the marks or pits being provided, in the form of heights and depressions, on the disk at the time of fabrication thereof. The depth of this recorded pit 35 for an ID signal is preset to be .lambda./4 or in the neighborhood thereof so that the pre-grooves 33 may not affect the track error signal.
Recorded information (user data) is recorded in the form indicated by a numeral 36 by means of a light beam incident on the recording layer 32 on the land portion 34 that borders on the recorded pit 35 for the ID signal, not as heights and depressions but as direction of magnetization of a magnetic film, the recording being done by using a mark length recording method (described later). Both in recording and reproducing, the light beam is incident from behind the substrate 31 and converged so as to be focused on the recording layer 32.
FIG. 3 shows how the magneto-optic signal is detected; and FIG. 4 shows a vector diagram of the components of the light reflected by the magneto-optic disk. As shown in FIG. 3, the plane of polarization of the light reflected by the magneto-optic disk 17 after the light is incident thereon is such that a reflected light A reflected from a part of the disk which is magnetized top to bottom rotates, due to the magnetic Kerr effect, in a positive direction, whereas the reflected light B reflected from a part of the disk which is magnetized bottom to top rotates in a negative direction, each rotation equaling .theta..sub.k degrees. .theta..sub.k denotes the Kerr rotation angle which has an extremely small value of about 1.degree..
The reflected lights A and B have an S polarized component and a P polarized component and, as shown in FIG. 4, may be represented by mutually different vectors. Therefore, in a readout system, the reflected lights are split with respect to the plane of polarization, i.e., are split into a P-polarized component parallel to the plane of incidence on the polarizing beam splitter 24 and an S-polarized component perpendicular thereto. Detection of the Kerr rotation angle .theta..sub.k is possible because the polarizing beam splitter 24 lets the P-polarized component pass through it and reflects the S-polarized component.
Since a 45.degree. offset is applied to the detected P-polarized component, by means of the wavelength plate 23, the detected P-polarized component is given by ##EQU1##
A description will now be given of how information is recorded on the magneto-optic disk 17. In an initial state, magnetization on the magneto-optic disk 17 is unidirectional, i.e., the disk is magnetized in a "erase" direction. In order to write a recorded data consisting of "0's" and "1's" arranged in a selected order on the disk, the disk is heated until the Curie point is reached (in the case of the magneto-optic disk, this means the Curie temperature, which is on the order of 100.degree.-200.degree. ; and, in the case of a phase transition disk, this means a phase transition temperature on the order of several hundred degrees) by means of a recording laser beam that is turned on and off in accordance with the recorded data. An external magnetic field is then applied to the heated portion of the disk, so that a series of marks, each having a generally elliptical shape, is formed, the magnetization direction of the masks being opposite to the "erase" direction.
Conventional methods of recording information on the magneto-optic disk include a mark position recording method (also known as a mark interval recording method), which is used in a rewritable optical disk, and a mark edge recording method (also known as a mark length recording method), which is used in a compact disk. Descriptions will now be given of the mark interval recording method and the mark length recording method with reference to FIG. 5.
Supposing that a recorded data sequence "0100100000001000" shown in FIG. 5(A) (the encoding of which data sequence is done in accordance with a (2,7) RLL (run length limited) formation suitable for recording information on the magneto-optic disk) is input when the mark interval recording method is implemented, recording marks (domains) P.sub.1, P.sub.2 and P.sub.3 are recorded on the disk positions corresponding to the data "1", as indicated by a shaded portion of FIG. 5(B), while no recording marks are recorded on disk positions corresponding to the data "0". That is, the mark interval recording method is a method in which the presence and absence of the recording marks is made to correspond to the recorded data "1" and "0" respectively. When data is retrieved from the optical disk in which the information is recorded in accordance with the mark interval recording method, data detection is effected by detecting the peak points in the reproduced waveform shown in FIG. 5(C).
In the mark length recording method, the edges of recording marks (domains) P.sub.10 and P.sub.11 are located on the disk positions corresponding to the data "1", as indicated by a shaded portion of FIG. 5(D), by reversing, at every position corresponding to the "1" bit in the above input data sequence, the recording light intensity.
When data is retrieved from the optical disk in which the information is recorded in accordance with the mark length recording method, data detection is made possible by binarizing the reproduced waveform shown in FIG. 5(E) with reference to a reference level (slice level). As can be seen in FIG. 5, the mark length recording enables higher recording density than does the mark interval recording. Given that the minimum mark length is the same, the mark length recording can achieve a recording density twice that of the mark interval recording.
Accordingly, it is found that the mark length recording method, in which the recorded data value "1" is made to correspond to the edge of the recording marks, is suitable for the purpose of improving the recording density of the optical disk. It is important, in the mark length recording method, to detect, at the time of reproduction, the position where the edge of the recording marks is recorded. Because the recording process of the optical disk is characterized as a heat mode recording, in which the laser light emitted from a semiconductor laser is used as a heat source in recording the recording marks, it becomes necessary to remove thermal shift and-pattern shift that occur during the recording process.
FIG. 6 shows thermal shift; and FIG. 7 shows thermal shift characteristics. A case is considered here in which the recorded data sequence shown in FIG. 6(A) is converted, in accordance with the mark length recording method, into the recorded pulse shown in FIG. 6(B), and the corresponding beam is impinged upon the optical disk, the high-level period of the recorded pulse being set to be substantially long, and the light intensity being set at a substantially high level. When the recording is effectuated such that the temperature at the position of impingement of the beam on the disk is raised so that reversal in magnetization direction therein occurs, the front edge of a mark P.sub.22, which, of the two adjacent recording marks P.sub.21 and P.sub.22, is the one recorded later, is recorded in a position removed, toward the preceding mark P.sub.21, by P.sub.t from where the front edge should otherwise have been recorded, due to thermal influence from the preceding mark P.sub.21.
This is how thermal shift occurs, and the shorter the interval between the preceding mark and the mark about to be recorded, in other words, the shorter the pulse interval (the low-level period of FIG. 6(B)) of the recording pulse, the greater the thermal shift amount .DELTA.P.sub.t, as shown in FIG. 6(D).
FIG. 7 shows the mark front edge shift occurring when the marks are recorded using a laser light at four write power levels ranging from 5.5 mW to 8.5 mW. In the case of 6.5 mW write power indicated by .quadrature., it is found that the shorter the interval between the marks, in other words, as the interval (in .mu.m) between the recording pulses for causing the LD to emit light becomes shorter starting from 2.5 .mu.m, 2.0 .mu.m till it is 1.0 .mu.m, the greater the mark front edge shift amount (in .mu.m) becomes. The same is true of the other write power levels.
FIG. 8 shows a pattern shift; and FIG. 9 shows pattern shift characteristic. A case is considered here in which the recorded data sequence shown in FIG. 8(A) is converted, in accordance with the mark length recording method, into the recording pulse shown in FIG. 8(B), the high-level period of the recording pulse being set to be substantially long, and the light intensity being set at a substantially high level, so that the heat mode recording like the one described above is effected and two adjacent marks P.sub.31 and P.sub.32 are formed on the optical disk, as shown in FIG. 8(C). Since the medium temperature is gradually raised when the marks P.sub.31 and P.sub.32 are being recorded, the positions at which the back ends of the marks P.sub.31 and P.sub.32 are recorded are shifted, further down the track, .DELTA.P.sub.c, .DELTA.P.sub.c +.DELTA.P.sub.p, respectively from positions where the marks should otherwise have been recorded.
This is how the aforementioned pattern shift occurs, and the longer the recording marks, in other words, the longer the pulse width (the high-level period of FIG. 8(B)) of the recording pulse, the greater the amount of pattern shift .DELTA.P.sub.p, as shown in FIG. 8(D).
FIG. 9 shows back edge shift occurring when the marks are recorded using a laser light at four write power levels, namely 5.5 mW, 6.5 mW, 7.5 mW and 8.5 mW.
In the case of 6.5 mW write power indicated by .quadrature., it is found that the longer the mark, in other words, as the duration (in .mu.m) of the pulse for causing the LD to emit light becomes longer starting from 1.0 .mu.m, 1.5 .mu.m till it is 2.5 .mu.m, the greater the amount of back edge shift (in .mu.m). The same thing is true of the other write power levels.
The above-mentioned .DELTA.P is a difference between: the length of the recording mark P.sub.31 manifested when the maximum pulse interval is followed by the pulse having the minimum pulse width; and the length of the same mark unaffected by pattern shift. .DELTA.P.sub.c is referred to as constant shift. This constant shift is characterized as a phenomenon in which the edge position varies in accordance with the variation of write power of the laser light. The variation of write power may be regarded as being equivalent to the variation of ambient temperature or the variation of sensitivity of the magneto-optic disk acting as a medium.
In order to successfully implement the mark length recording method, it is essential to remove, in a recording process, thermal shift .DELTA.P.sub.t and pattern shift .DELTA.P.sub.p, from among the above-mentioned three kinds of shift, i.e., thermal shift .DELTA.P.sub.t, pattern shift .DELTA.P.sub.p and constant shift .DELTA.P.sub.c. This is because, while constant shift .DELTA.P.sub.c is a component subject to variation due to medium sensitivity or ambient temperature and may be compensated for, because of a prevalent low-frequency component, in a signal processing system, the other two kinds of shift .DELTA.P.sub.t and .DELTA.P.sub.p may be expected to contain much the same frequency component as the recording signal, and are difficult to eliminate at the time of reproduction. Accordingly, the above edge shifts are dealt with by affecting recording compensation such that the timing at which the LD is turned on and off is varied in accordance with the recorded data pattern so that the accurate reproduction may be achieved. Specifically, it was reported by the Institute of Electronics, Information and Communication Engineers that compensation for the timing of turning the LD on is required to eliminate thermal shift; and compensation for the timing of turning the LD off is required to eliminate pattern shift.
It is proposed that the variation of medium sensitivity be dealt with by performing trial recordings for a plurality of test regions along the radius of the disk, wherein the write power level used at the time of erasing, recording and reproducing is varied as a parameter, and that a combination of write power levels causing the fewest errors when the data is being reproduced be used thereafter in performing recording and reproducing.
Conventional proposals for executing recording compensation with respect to edge shift include the methods as described in the Japanese Laid-Open Patent Publication No. 63-53722, No. 63-281229 and No. 62-12463, in which the densest data pattern or a predetermined data pattern is detected so that the recording pulse width is controlled at the time of recording.
These methods described in the Japanese Laid-open Patent Publications are based on the controlling of the pulse width in accordance with the data pattern. A simplified description of these methods will be given by describing an example of the Japanese Laid-Open Patent Publication No. 63-53722, with reference to the circuit diagrams of FIG. 10 and 11.
In FIG. 10, a D flip-flop 41 forms a mark edge recording data pattern (NRZ code) on the basis of a given code, and a delay is applied to the NRZ code by means of a delay element 42. Meanwhile, a correction amount is determined by a recording corrector 43 on the basis of control information; and, on the basis of this amount, a delay amount applied by the delay element 42 is selected by a selector 44. Specifically, a signal from one of a plurality of output taps of the delay element 42 is selected. An AND gate 45 generates a data pulse for which the pulse width is corrected on the basis of a delayed data pulse and a non-delayed data pulse. The generated data pulse drives a semiconductor laser driving system 46. Further, laser power is controlled such that a correction amount is transmitted from the recording corrector 43 to a D/A converter 48 of a power setting unit 47 so that the driving of the semiconductor laser is controlled in accordance with the data pattern.
FIG. 11 is a diagram of a circuit for generating control information of FIG. 10. In FIG. 11, a counter 49, an AND gate 50 and a D flip-flop 51 detect a pulse (HIGH PW-P) having a predetermined pattern, on the basis of a data pulse (DATA-P) and a clock pulse (CK-P), so that a power control signal is introduced in the circuit. Further pattern detection is executed by a counter 52, which is fed an output signal from the D flip-flop 51, and an AND gate 53. A delayed data pulse (DELAY DATA-P) is created by delaying the data pulse (DATA-P) by means of a shift register 54. The pulse (HIGHPW-P) and the delayed data pulse (DELAY DATA -P) are sent to the recording corrector 43 of FIG. 10 as the control information.
The present inventors would like to emphasize, with reference to FIGS. 7 and 9, the fact that pattern shift is determined solely by the recording mark length, and thermal shift is determined solely by the preceding pulse interval.
Accordingly, the method of detecting the pulse width and controlling laser power as used in the above-described apparatus functions such that the control amount at the rise and that at the fall of the recording pulse are the same. Such a method has a disadvantage in that edge shift is not completely corrected upon the introduction of only one control amount, because pattern shift and thermal shift are mutually independent and asymmetric, as described above.
As shown in FIG. 10 and 11, the method, in which the recording data pattern is detected and a amount delay amount is determined therefrom, has a disadvantage in that the circuit configuration becomes complex and the circuit becomes too large, and in that the timing for controlling becomes complicated, as the number of patterns to be detected increases. Moreover, the above method has a disadvantage in that implementing real-time control of a semiconductor laser power in accordance with the data pattern results in an increase of control parameters as well as an excessively complex circuit configuration.
The aforementioned method of performing trial recordings to deal with the variation of medium sensitivity from one medium to another has a disadvantage in that determination of a recording compensation amount with respect to edge shift characteristics is not taken into consideration in such trial recordings, in spite of the fact that not only the variation of medium sensitivity from one medium to another but also the variation of edge shift characteristics and compatibility with mediums having different edge shift characteristics need to be considered, and in that, consequently, proper recording is not executed.